As radio circuits become more complex and also are designed to work at still higher frequencies, the elements tend to consume more power. Frequency dividers are important elements of radio circuits, and are for example used for synthesizing signals of desired frequency and phase. For example, multi-band radio circuits rely on the ability to generate signals at different controllable frequencies. Another example is application of beamforming where a plurality of antennas is fed by signals with a controlled phase such that the arrangement of the plurality of antennas provides a desired directional characteristic.
It is foreseen that cellular systems may use millimeter waves. The frequencies may in such cases range from about 15 to 60 GHz. In order to use the system outdoors, a longer cyclic prefix may be used compared with 60 GHz indoor systems. For this, a closer sub-carrier spacing in the OFDM modulation may be advantageous, however posing stringent phase noise requirements. At the same time beamforming is advantageously supported to increase the range and capacity of the system. A large number of antenna elements are then used. The signal at each element will have an individual phase shift which controls the beam direction. One key implementation alternative is to impose phase shifts in the local oscillator signal. The local oscillator frequency is in such cases preferably made programmable to be able to operate on different channels and in different bands. Individual local oscillators can then be placed in close vicinity to the antenna element making local oscillator phase noise between antennas uncorrelated. One alternative for the generation of quadrature phase local oscillator signals used for single side band up-/down-conversion is to use frequency dividers.
An implementation of the local oscillator generation circuitry beneficially strives towards achieving low phase noise, individually programmable phase, programmable frequency, and/or distributing the signals to all transceivers in a beamforming system, all without consuming excessive power.
Quadrature frequency dividers, able to generate signals with 90 degrees phase shift may have two different modes of operation. The two modes have their output phases shifted 180 degrees with respect to each other. The actual mode of operation depends on the initial state of the divider whereof each of the two possibilities ideally has 50% probability. In a multi-antenna system, combining antenna streams down/up converted with quadrature local oscillator signals 180 degrees shifted will not result in constructive combining of the signals as intended. In a receiver where each signal path is converted to digital domain and then combined, this could be compensated for in the digital base-band, but if combining is to be done in analog domain, this needs to be corrected for before combining. In a transmitter this also needs the same attention in analog domain.
Direct detection of the actual phase relationship of the LO-signals could be done, but would require great care, especially at mm wave frequencies, to really make sure that the signal comparison is made correctly without losing the information when distributing the signals to the point of comparison. If direct sampling of a signal above 10 GHz is to be done, there is a 50 ps window to perform the sampling, which would require great care when routing the sampling signal over an antenna array, possibly covering several square centimeters.
The initial state of the frequency dividers could be controlled, although not trivially, but would require the individual synthesizers to settle in identical ways. Thus, this is hard considering mismatch between them.
It is therefore a desire to provide an approach for an electronic circuit to alleviate this.